Lightweight composite truss wind turbine blade

ABSTRACT

A lightweight wind turbine blade formed with a truss support structure assembly of composite truss joints including composite spar and cross members attached to and supporting in spaced relation a spine of lightweight rib panels. The rib panels are oriented in parallel spaced relation from one another and individually molded with perimeters defining individual areas of curvature for the finished blade assembly. The truss support structure is covered with a lightweight fiberglass or hardened fabric skin attached to and fitted on respective rib panel edges forming an airfoil structure.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a continuation application of commonly assigned U.S.patent application Ser. No. 11/725,916, filed on Mar. 20, 2007 now U.S.Pat. No. 7,517,198, and entitled LIGHTWEIGHT COMPOSITE TRUSS WINDTURBINE BLADE, which claims priority of U.S. Provisional PatentApplication Ser. No. 60/783,551, filed Mar. 20, 2006.

FIELD OF THE INVENTION

The invention relates to airfoil core structures and specifically, windturbine blades.

BACKGROUND OF THE INVENTION

With populations increasing and newly developed communities springingforth in previously uninhabited locales, the demand for power is testingthe capacities of energy providers. New sources of fuel or power arebeing sought daily and sources previously considered inefficient to tapare being studied further to extract whatever power generation may beavailable. However, fossil fuels such as coal and petroleum, previouslyfavored because of their abundance and inexpense are now becomingshunned because of the presumed deleterious effects their consumptionhas effected on the environment and their increasing costs.Additionally, there are some that feel the United States has becomedependent on the production of fuel from foreign nations and thus, weshould seek alternatives to these fuel sources. In response, industriesare converting more machinery previously powered by petroleum-basedproducts to electric-based power sources. Additionally, the populationas a whole is becoming increasingly dependent on electrical equipment tomanage our businesses, transport our workforce, and run our homes. Thus,the search for alternative energy sources is gathering increasinginterest with the hope that natural power sources will provide enoughclean energy to power our nation's increasing energy demands. One sourceexpected to help meet the need for meeting the for energy production iswind power.

Wind power is the conversion of wind energy into more useful forms suchas electricity. Wind energy is considered by many an ample, renewable,widely distributed and clean power source that mitigates the greenhouseeffect if used to replace fossil-fuel-derived electricity. Wind power isfor the most part relegated to large scale wind farms for use innational electrical grids. Small individual turbines are used forproviding electricity to rural residences or grid-isolated locationsbecause of the current structural capabilities and the economicobstacles associated with generator manufacture and territorialplacement.

Most major forms of electric generation are capital intensive, meaningthey require substantial investments at project inception but have lowongoing costs (generally for fuel and maintenance). This is particularlytrue for wind power which has minimal fuel costs and relatively lowmaintenance costs. However, wind power has a high proportion of up-frontcosts. The “cost” of wind energy per unit of production is generallybased on average cost per unit, which incorporates the cost ofconstruction (including material components), borrowed funds, return toinvestors, estimated annual production, among other components. Thesecosts are averaged over the projected useful life of the equipment,which can be in excess of twenty years if the generator equipmentmaintains durability and efficient production. Thus, minimizing the riskof premature breakdown while extracting the most power from a givenlocale becomes a compelling goal when fabricating a wind powergeneration source. One of the most common and widely used structures forwind power extraction is a wind turbine.

A wind turbine is a machine the converts kinetic energy from the windeither into mechanical energy used directly by machinery such as a pumpor is then converted into electricity which is subsequently used topower electric equipment. Wind turbines are popular sources of powerbecause they do not rely on the burning of fossil fuels whoseconsumption is a known contributor to the pollution of the environment.Wind turbines are commonly separated into two types: horizontal axiswind turbines or vertical axis wind turbines. For this application,discussion will focus on a wind turbine blade for use on a horizontalaxis wind turbine. Such wind turbines have a main rotor shaft andelectrical generator at the top of a tower and are pointed into thewind. Common modern wind turbines are pointed into the wind andcontrolled by computer-controlled motors. The blades should be madestiff and strong to resist bending, shear, and torsional forces producedby strong winds. Horizontal axis wind turbines are popular amongstenergy harvesters because the design of the blades and their placementare conducive to self starting and operation whenever the blades aresubjected to winds.

In practice, wind generators are usually sited where the average windspeed is 10 mph or greater. An “ideal” location would have a nearconstant flow of non-turbulent wind throughout the year and would notsuffer from excessive sudden, powerful wind gusts. Current preferredsites include windy areas such as hilly ridgelines, shorelines, andoff-shore developed platforms situated in shallow waters. However, animportant turbine siting consideration is access to or proximity tolocal demand or transmission capacity and such typical sites are distantfrom local demands; especially those growing demands created byburgeoning communities in flat, low wind-speed areas. Low wind-speedareas have wind power potential, however, the current technology isconsidered by some inefficient and/or cost prohibitive for use near tothese locales.

During the general operation of a wind turbine, the air that passes overthe upper camber of an airfoil must travel faster than the air travelingunder the lower camber. Thus, a difference in pressure is formed wherethe air traveling over the upper camber is at a lower pressure than theair traveling under the lower camber. This results in a lift force onthe blade, which induces a torque about the rotor axis, causing theturbine to rotate. Thus, energy is extracted from this torque on thewind turbine blades.

Several factors contribute to the efficiency of a wind turbine system.The one important factor is the length of the blades, as the total powerthat can be extracted is proportional to the disk area swept by therotor blades as they rotate, which is proportional to the square of theblade length. Other factors include the ability of the control system tomaintain the optimal tip speed ratio. Factors such as low blade weightand low rotational inertia of the rotor make it easier for the controlsystem to maintain the ratio between wind speed and blade rotationspeed, increasing and decreasing the rotor speed as wind speedsfluctuate.

One obstacle to the development of longer wind turbine blades necessaryto increase the disk area and power production is the rapid increase inblade weight as the blade length increases. As blade length increases,the loads on the blade increase rapidly. A longer blade is also moreflexible than a shorter blade. In order to resist the increased loadsand to provide the required stiffness, a significant amount ofadditional material must be added to longer blades to maintainstructural integrity. The addition of material increases blade materialcost, as additional material must be purchased and processed. Additionalweight in a wind turbine blade is detrimental because it increasescertain loads on the hub and generator systems due to the increasedrotational inertia of the rotor disk, and the increased gravity loads onthe blades. Furthermore, additional weight can be detrimental because itcan cause a reduction in the natural vibration frequencies of theblades, potentially causing undesirable interactions with the airstreamand/or the dynamics of the tower and support structure.

Therefore, any method making more efficient blade structure has thepotential to reduce the material cost, and to allow larger blades to bebuilt. In some cases, these larger blades may be combined with existinggenerators, allowing additional power to be generated, especially in lowwind speed areas. This is important, as a large fraction of the UnitedStates has relatively low wind speeds. Furthermore, since the wind speedat a given location varies with time, the use of a larger blade maylower the minimum wind speed at which a turbine can be profitablyoperated, allowing turbines at a given site to be generating power alarger fraction of the time. This can result in a significant reductionin the overall cost of energy from wind turbines.

For instance, some of the first wind turbines were constructed of woodand canvas sails because of their affordability and easy construction.However, wood and canvas materials require a lot of maintenance overtheir service life. Also, their shape was associated with a lowaerodynamic efficiency creating a relatively high tried for the forcethey were able to capture. For these reasons, wind turbine blades werereplaced with a solid airfoils structures.

Other older style wind turbines were designed with relatively heavysteel components in their blades (such as steel girders, cross bars, andribs), which produced a higher rotational inertia. While improved inaerodynamic efficiency, structural durability and maintenance, the speedrotations in heavy steel blades required governance by an alternatingcurrent frequency of the power source to buffer the changes in rotationspeed to thus, make power output more stable. Furthermore, the weight ofsteel becomes economically prohibitive in designing longer bladescapable of rotating in large arcs within the low-speed wind areas.

Subsequent methods of forming wind blade airfoils involved usingaircraft construction techniques. These techniques included using heavybalsa wood laid across the main metal or wood bar of a blade runningdown the length of the blade. Many of these types of blades used a setof ribs providing chord wise support and maintaining airfoil shape.Skins of sheet metal were riveted to the rigid ribs therein to providethe aerodynamic surface. While lighter than primarily steel blades,these designs still suffer from the shortcomings associated with theeconomics of weight per blade unit length of components.

Currently, wind turbine blade fabrication mimics the same techniquesused in boat building and surfboard construction. Some currentconventional wind turbine blades are manufactured at a lengthapproximately 100 to 150 feet long. Materials of choice are commonlyfiberglass with epoxy resin forming airfoils using wet layup techniques.The blades are fabricated in large costly “clamshell” molds where skinsand heavy glass balsa panel cores are laid up manually. Such solidfiberglass structures are relatively heavy for a 31 meter blade(approximately 12,000 pounds) and require expensive tooling forfull-scale heated molds.

Other more sophisticated techniques include a turbine with blades thatcan be twisted in response to variable torque forces. A device of thistype can be seen in U.S. Pat. No. 5,284,419 to Lutz.

It can be seen therefore, that a need exists in the art for a windturbine blade made from sturdy construction capable of withstandingsudden wind loads yet, is lightweight, economical, and materiallyefficient for production in longer lengths capable of generating powerin low wind speed areas. Additionally, a need exists for such a bladethat can be readily disassembled for shipment in standard transportationcontainers and readily assembled on site.

SUMMARY OF THE INVENTION

Briefly and in general terms, the wind turbine blade of the presentinvention includes an internal truss support structure comprising a setof ribs with perimeter edges including flanges and attachment fixturepoints, the ribs aligned in parallel and laterally spaced from oneanother on their edge forming a spine. The ribs are connected togetherby composite spar and cross members. The spar members are attached alongthe spine to the perimeter edges of respective ribs along the attachmentfixture points. Likewise, the cross members are bonded between adjacentribs with at least one cross member passing through the gaps betweenadjacent ribs and attached to spar members at respective attachmentfixture points forming a series of truss joints. The truss supportstructure is then covered by an airfoil skin attached onto to theflanges of the ribs.

Other features and advantages of the invention will become apparent fromthe following detailed description, taken in conjunction with theaccompanying drawings which illustrate, by way of example, the featuresof the invention

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partially exploded perspective view of a first embodiment ofthe wind turbine blade illustrating the internal truss supportstructure;

FIG. 2 is a perspective view of the truss support structure shown inFIG. 1;

FIG. 3 is an enlarged sectional view of the circle 3 shown in FIG. 2;

FIG. 4 is a top view of the truss support structure shown in FIG. 2;

FIG. 5 is a front view of the truss support structure shown in FIG. 2;

FIG. 6 is an enlarged sectional view representation of a portion of asecond embodiment of the truss support structure shown in FIG. 2;

FIG. 6A is an enlarged sectional perspective view of a splice jointshown in FIG. 6;

FIG. 7 is an enlarged sectional perspective view of a joint shown in thetruss support structure shown in FIG. 6;

FIG. 8 is an enlarged sectional front view of a joint shown in the trusssupport structure shown in FIG. 6;

FIG. 9 is a sectional perspective view depicting a wind turbineincorporating the present invention;

FIG. 10 is an enlarged sectional front view of the invention shown inFIG. 9;

FIG. 11 is a block diagram describing a method of fabricating theinvention shown in FIG. 1; and

FIG. 12 is a perspective view of an example assembly fixture describedin the diagram of FIG. 11.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

There are some landscapes where the windspeed is too low for some windturbine blades to harvest the surrounding air. In many instances, theblades are unable to harvest the air and rotate about their turbine axisbecause the blades are too heavy and/or to short to effectively capturerelatively low-speed winds. At the heart of some overweight andundersized wind turbine blades is a core support structure that isinefficiently designed to provide weight and low rotational inertiasuitable for capturing energy in low wind speed areas. As describedherein, Applicants have invented a new wind turbine blade utilizing acore support structure which provides a blade of reduced weight whichstill meets the structural needs or requirements of a turbine blade.

As shown in FIG. 1, the wind turbine blade 100 of the subject inventionincludes a relatively lightweight airfoil that exploits the efficiencyof a substantially lightweight composite support truss structure 20which extends axially lengthwise from a root attachment section 60 to atip 70. The support truss structure 20 is covered by an assembly ofskins 90 forming the basic airfoil shape of the blade 100. The skins 90include in the chord-wise direction of the blade 100 a leading edge 74and a trailing edge 76. The upper camber of the blade 100 is designated75 and the lower camber is designated 77. In an exemplary model, theassembled blade 100 is approximately 31 meters from root attachmentsection 60 to tip 70, however, it is understood that the blade can belengthened according to the needs of the wind generation site.

As seen in FIG. 2, the core of the wind turbine blade is comprised of asupport truss structure 20 that is formed by a series of laterallyspaced ribs 30 that are oriented generally parallel to the broad face 34of one another along the longitudinal lengthwise axis of the windturbine blade 100 forming a spine 25 and defining the general airfoilshape. The ribs 30 are connected in support to one another by aplurality of connected composite spar members 40 and cross members 50commencing attachment and support from a first rib 30 at the rootattachment section end 60 and terminating at an end rib 30 located atthe tip 70. As seen in detail in FIG. 3, a first preferred embodimentincludes four spar members 40 join the series of ribs 30 together attheir outer perimeter edges 36 connected to the ribs at rib bondingfixtures 35, providing longitudinal support to adjacent ribs. Connectingin support therebetween are a plurality of cross members 50. Referringto FIGS. 3-5, the cross members 50 essentially comprise four memberscrossing between adjacent ribs 30 with two of the cross membersconnected to adjacent ribs in the chord-wise direction while the twoother members support attachment to adjacent ribs in the camberdirection of the spine 25.

Those skilled will recognize that the ribs 30 provide the basic airfoilshape of the blade 100. The ribs 30 are fabricated preferably lightlyloaded with a lightweight material such as a balsa core sandwiched oneither side with another light weight material such as fiberglass andformed as flat panels approximately one inch thick with flanges 37incorporated around their perimeter. In the exemplary blade model, theribs 30 are spaced parallel approximately 1 meter apart providing theprimary support for the skin panels 90 bonded to the rib perimeter edges36 on the flanges 37 for attachment of the skins 90 thereto. It will beunderstood that as different portions of blade surfaces changecurvature, the individual ribs 30 are molded to the shape and supportnecessary for each segment.

The spar members 40 and cross members 50 are fabricated of cylindricallypultruded composite material pre-cured when assembled. Truss joints 55are formed where respective spar members 40 and cross members 50intersect at respective bonding fixture points 35 along the perimeteredge 36 of individual ribs 30. As seen in FIGS. 2-5, spar members 40 runcontinuously and essentially longitudinally straight along the perimetertangent of the spine 25 from root attachment section 60 to tip 70.Likewise, cross members 50 comprise four continuously running membersweaved diagonally across the longitudinal axis of the spine betweenadjacent ribs 30 in the chord and camber directions of the spine 25connected to respective truss joints 55 from root attachment section 60to tip 70. In a blade measuring approximately 31 meters, approximately120 joints are formed using such a configuration.

Referring to FIGS. 6-8, a second truss formation is shown. In thissecond preferred embodiment, the truss support structure 20 is modifiedby reconfiguring the cross members 50 to cooperate with the spar members40 forming a series of triangular truss joints 57. In this embodiment,the cross members 50 are re-designated individually as diagonal members50 d and as vertical members 50 v for the sake of descriptivesimplicity, but are for all intents and purposes are materiallyequivalent to the cross members 50 as shown in FIGS. 1-5. It will beunderstood that member 50 v refers to those cross members that areconnected vertically between vertically adjacent spar members 40 andcross members designated 50 d refer to those cross members thatintersect and diagonally join two adjacent cross members 50 v.

Referring specifically to FIG. 6, the drawing depicts a section of thespine 25 showing a parallel set of three consecutive truss joints 57where a rib is represented by the rib plane RP. The section of spine 25shown in this embodiment is similar to the embodiment shown in FIGS. 1-5except that the truss structure 20 is configured to form two parallelseries of vertical joints from root attachment section 60 to tip 70. Itwill be understood according to this drawing that a rib would beattached to parallel running vertical members 50 v on their flanges 56.In this embodiment, an alternate joint embodiment is formed byincorporating an alternative truss joint 57 of individual members 50 v,50 d and 40. If necessary, additional diagonal members could be addedattaching the leading truss section to the tailing truss section.

Unlike the first embodiment, the cross members 50 v and 50 d are formedas individual and potentially interchangeable components disconnectedfrom like components along the length of the truss structure 20. In thisembodiment, two joints 57 are formed between a pair of ribs when adiagonal member 50 d intersects the upper end of a vertical member 50 vand the lower end of an adjacent vertical member 50 v connected alongthe same set of upper and lower spar members 40. Referring to FIGS. 7and 8, spar members 40 include a U-shaped channel 46 running from end toend with a width large enough to accommodate the end of a verticalmember 50 v. One end of respective vertical members 50 v incorporate aslot 54 accommodating the end thickness of an intersecting diagonalmember 50 d. Individual composite support members further include bores44 centrally aligned at the joints 57 for insertion of a bolt or pin 42.The joint 57 can be further strengthened by inserting metal plates 48within the channels 46 on both sides of a vertical member 50 v. Thevertical members 50 v further include a flange 56 on the member sidesfor bonding receipt of respective ribs to the flanges.

It will be appreciated that as individual and potentiallyinterchangeable components, the ribs 30, vertical members 50 v anddiagonal members 50 d, and skins 90 can be separated for storage andtransported in a standard 40 foot shipping container and that the blade100 parts lend themselves to efficient disassembly and reassembly onsite. Referring to FIGS. 6 and 6A, when desirable, the truss supportstructure 20 can include modified spar members 40 that are constructedin intervals less than 40 feet long to accommodate transportation of adisassembled blade 100. When assembling the spar members 40 to the ribs30, structural unity and integrity can be preserved by forming splicejoints 45 at the ends of respective spar members 40 intermediate anytruss joints 57. In such instances, respective spar member ends includea plurality of bores 43 in the spar member side walls aligned from oneanother across the U-shaped channel 46. A bridging member 48 made fromsteel or other similarly strong material is press fit within theU-shaped channel 46 traversing and splicing together the respective endsof two axially adjacent spar members 20. The bridging member 48 alsoincludes bores 43 aligned with respective bores in the spar members 40.The respective spar member ends are then secured and bonded together byinserting pins 44 through respective aligned bores of the spar 40 andbridging members 48 forming the splice joint 45. The size of thebridging member 48 is kept to a minimum preserving the lightweightfeatures of the truss structure 20 while maintaining the load capacitieson the sectionalized spar members 20. Additionally, the spar members 40are terminated in lengths such that their splicing together is performedat predetermined distances intermediate from any truss joints 57 so asto preserve respective joint integrities.

Once assembled, the turbine blade 100 affords a sturdy yet lightweightairfoil structure readily attachable to commercial wind turbines 99 asseen in FIGS. 9 and 10. The skin panels 90 are constructed of flat-panelfiberglass which can be manufactured in bulk and shaped therefrom oralternatively, the truss structure 20 can be covered in a fabric fittedover the areas of varying curvature which fabric can then be hardenedwith a resin to finish the blade airfoil. In constructing the blade, theskins 90 are wrapped around the truss structure 20 and fastened to theribs 30 on each rib perimeter edge 36 to the flanges 37, (FIGS. 1-3).Those skilled will understand that any conventional bonding techniquesuch as adhesive bonding or the use of mechanical fasteners are suitablefor attaching the skins 90 to the ribs 30. Additionally, it will beappreciated that where the ribs 30 define areas of high curvature, theskin panels 90 can span across a single pair of ribs. However, in areasof low curvature, several ribs can be covered by a larger section of theskin panel 90. Once the skins 90 are mounted and bonded to the trussstructure 20, the skin 90 behaves as a single continuous skin and hencemay support the internal components of the blade 100 as a unitarystructure that can be mounted from the root attachment end 60 fortransferring wind loads to a hub 72 (FIGS. 9 and 10).

The root attachment end 60 maintains compatibility with current windturbine hubs by including a steel mounting ring 80 incorporating acollar 85 with four hard points 87 projecting outward from the collar.The four spar members 40 are attached to the interior of the hardpoints. The skins 90 are draped over the spar members 40, hard points 87and collar 85. The assembled section end 60 is secured onto the windturbine 99 by bolting the mounting ring 80 to the hub 72 using bolts.Once mounted, the spar members 40 are the primary load carriers whilethe continuous skin 90 at the root attachment end 60 carries the loadfor transfer to the hub 72.

In operation, as wind comes across the blade 100 surface, the wind forceacts on the blade producing a torque about the hub causing the blade torotate. As the blade 100 rotates, shear and bending forces act on theblade as it continues through its circular path. Those skilled willrecognize that as forces act on the blade, the spar members 40 willcarry the bulk of the bending force loads while cross members 50 willprovide the shear and torsional support. Thus, the truss structure 20described can be either utilized to produce a lighter blade of standardlength (e.g. approximately 100 feet), or can be used to produce longerblades with weight comparable to current designs which can harvest windfrom a greater arc area of rotation.

It will be appreciated that by using composite components in the trussstructure 20, sufficient strength performance and fatigue resistance isachieved while providing rigid support of the blade in a relatively lowdensity structure. By using composite spar members 40 and/or crossmembers 50, such lightweight components constitute the primary loadpaths in truss joints 55 in cooperation with laterally spaced ribs 30.Additionally, by using a fabrication that results in highlyunidirectional to fabricate the support members, the fibers are alignedstraight along the axial direction of fabrication, thus providingreinforcement and homogenous strength within the member. Thus, asupported and reinforced structure is described that provides relativelyample sections of empty space within the blade 100 producing lessmaterial weight per unit length. Overall, the weight of a 100 foot bladeusing the proposed truss structure 20 could weigh as little as 6,000pounds (compared to the 12,000 pounds of the current conventionalfiberglass blade) resulting in up to a 50% weight reduction over somefiberglass blades. It will be understood that where reinforcement of theblade is desired additional cross members 50 and spar members 40 can beadded for load path redundancy without significantly contributing to theoverall weight of the blade 100.

Also, current conventional fiberglass blades are considered by some tooheavy and inefficient to generate power when fabricated at lengths above100 feet. In contrast, the weight per unit length of the blade 100 isrelatively light, and the wind mill 99 can also benefit from blades 100constructed of longer lengths which can cover a greater sweeping arcarea of wind while maintaining lesser weight loads. In areas of low windpower, longer blades rotating in a greater arc may harvest more wind andtherefore generate more power. It will also be appreciated that thelight weight of the blade 100 exerts less of a weight load on the hub72, and thus, less stress is placed on the hub, turbine, bearings, andtower. Wind mills 99 incorporating blades 100 will be expected toproduce up to five megawatts of power while operating at a net cost ofenergy reduction on the order of 30%-40%.

The proposed invention also lends itself to an economic process formanufacturing wind turbine blades. Referring to FIGS. 11 and 12, aprocess is described that incorporates selecting a congregation of ribs30 aligned to define the shape of the blade. Composite parts arefabricated and pre-cured before assembly. A set of posts 88 are spacedon the ground or on a fixture to a predetermined distance and the ribs30 are mounted to the posts. Composite spar members 40 are then affixedfrom the root to the tip along the rib perimeter edges 36 to mountingfixtures 86 and 87. Then, beginning from the root and moving out to thetip, composite cross members 50 are attached between upper and lowerrunning spars 40 forming the truss joints 55. A skin is then applied tothe truss structure 20 finishing the blade. The skins can be fabricatedfrom thin biaxial prepreg fiberglass (approximately 2 plies) cut intoshape or by applying a fabric over the structure which is then heatshrunk to an aerodynamic fit and coated with a resin.

As described herein, the blade 100 of the present invention demonstratesa new and useful structure that provides a lightweight sturdyconstruction useful for producing energy efficient power generation.

1. A wind turbine blade, comprising: a wind turbine blade skin having alow pressure surface facing in a first direction and a high pressuresurface facing in a second direction opposite the first direction; aplurality of spaced-apart wind turbine blade rib panels positionedwithin the skin; multiple spars extending between neighboring rib panelsin a spanwise direction, wherein at least one of the spars is curved intwo directions transverse to the spanwise direction; and multiple crossmembers extending diagonally between spars, with the ribs, spars andcross members forming a truss structure.
 2. The wind turbine blade ofclaim 1 wherein the at least one spar is curved in a chordwise directionand in a camber direction.
 3. The wind turbine blade of claim 1 whereinthe at least one spar is one of four spars positioned within the windturbine blade skin.
 4. The wind turbine blade of claim 1 wherein atleast one of the spars has a first portion connected to a second portionat a spar joint.
 5. The wind turbine blade of claim 1 further,comprising: a circular mounting ring; and a transition structureconnected between the truss structure and the mounting ring, wherein thespars are attached to the transition structure.
 6. A wind turbine blade,comprising: a wind turbine blade skin; a plurality of wind turbine bladeribs positioned within the skin; a plurality of spars extending betweenneighboring ribs in a spanwise direction, at least one of the sparshaving a first portion connected to a second portion at a spar joint;and a plurality of cross members extending diagonally between the spars,with the ribs, spars and cross members forming a truss structure.
 7. Thewind turbine blade of claim 6 wherein the spar joint includes a splicejoint.
 8. The wind turbine blade of claim 6, further comprising abridging member attached to both the first and second portions of the atleast one spar at the spar joint.
 9. The wind turbine blade of claim 8wherein the bridging member is bolted to the first and second portions.10. The wind turbine blade of claim 6 wherein the first and secondportions of the at least one spar are connected with bolts.
 11. The windturbine blade of claim 6 wherein individual cross members are attachedto individual spars at truss joints and wherein the spar joint islocated between and spaced apart from neighboring truss joints.
 12. Thewind turbine blade of claim 6 wherein spar joint is located between andspaced apart from neighboring wind turbine ribs.
 13. The wind turbineblade of claim 6 wherein an individual rib includes a recess and whereinan individual spar is received in the recess of the rib.
 14. The windturbine blade of claim 13 wherein an outer surface of the individualspar is generally flush with an outer surface of the individual rib. 15.The wind turbine blade of claim 6 wherein the skin includes multipleskin sections carried by the truss structure.
 16. The wind turbine bladeof claim 6 wherein at least one of the cross members includes a slot anda through-hole that extends generally transverse to the slot, andwherein the cross member is coupled to the rib with a fastener thatpasses through the through-hole.
 17. A wind turbine blade, comprising: awind turbine blade skin having a tip region and a root attachmentregion; a plurality of wind turbine blade ribs positioned within theskin; a plurality of spars extending between neighboring ribs in aspanwise direction; a plurality of cross members extending diagonallybetween the spars, with the ribs, spars and cross members forming atruss structure; a generally circular mounting ring; and a transitionstructure connected between the truss structure and the mounting ring,wherein the spars are attached to the transition structure.
 18. The windturbine blade of claim 17 wherein the transition structure includes acollar attached to the ring, the collar having reinforced regionsprojecting away from the ring, and wherein the spars are attached to thehard points.
 19. The wind turbine blade of claim 18 wherein thereinforced regions include hard points.
 20. The wind turbine blade ofclaim 17 wherein the plurality of spars includes four spars.
 21. Thewind turbine blade of claim 17 wherein the mounting ring includesmounting holes positioned to bolt the mounting ring to a wind turbinehub.
 22. The wind turbine blade of claim 17 wherein at least part of thetransition structure tapers as it extends from away from the spar andtowards the mounting ring.
 23. The wind turbine blade of claim 22wherein the tapering part of the transition structure includes afan-shaped portion that has an increasing circumferential extent asextends away from the spar and toward the mounting ring.